A major objective of the present proposal is to determine the detailed structure of the envelope proteins of Rauscher murine leukemia virus (R-MuLV). We will determine the origin of the minor viral glycoprotein, gp52, and its possible relationship to the major glycoprotein, gp70. The nature of the interactions of envelope proteins with one another and with the underlying membrane will be explored by several approaches including protein cross-linking experiments and the use of various dissociating conditions. These studies will be extended to analysis of the carbohydrate components of R-MuLV glycoproteins and the linkage of sulfate to these glycoproteins. We also proposed to obtain new information on the fine structure of arenavirus particles, and to characterize events in the replication and assembly of the arenaviruses Pichinde, Tacaribe, and Tamiani. We will carry out electron-microscopic studies of the arrangement of the internal ribonucleoprotein and the surface glycoproteins. We will attempt to determine the number of glycoprotein molecules in the spike structure, and whether the two glycoproteins of Pichinde virions are present in the same or different spike structures. The interactions between viral components will also be explored as described for R-MuLV. Analysis of the synthesis of arenavirus polypeptides will be directed toward detecting possible cleavage events and control processes in the formation of virion proteins. We will also attempt to identify virus-coded nonstructural polypeptides in infected cells. A series of ts mutants of Pichinde virus will be used to further analyze the process of virus assembly in electron-microscopic studies.